Identify this noise.

February 8, 2006 8:58 PM

Sound hounds, tell me: what's the source of my AM radio noise? [Warning: MP3 link]

It sounds like a very particular type of interference, so I'm hoping it's identifiable by noise alone. It sounds to my ears like the noise generated by a dimmer switch at one of the intermediate settings.

It's all over the shortwave bands, gets worse at lower frequencies, and disappears only when I unscrew the antenna jack from the back of the receiver. (Basic specs: outdoor aluminum loop antenna, about 15 feet into the middle of a driveway surrounded on all sides by condos. Radio is grounded to a metal pipe. No dimmers or appliances in my apartment are the source, so could be anything or anyone else...) Also no visible overhead power lines, although the condos do tower over the antenna rather considerably, as you might imagine.

It does seem to change its frequency spread during the day, but it's still very much there even in daylight hours.

Ideas?

posted by mykescipark to technology (12 comments total)

That's gotta be a Spy Number Station. The guy at the end has a definite Russian accent
posted by tellurian at 10:29 PM on February 8, 2006

That sounds great! Could you set up a stream please? :D
posted by aubilenon at 10:41 PM on February 8, 2006

Near the end I think I hear the second ticks and minute beep of NIST's time broadcast from WWV (MP3 sample, from here).
posted by RichardP at 10:58 PM on February 8, 2006

are the radio and antenna grounds tied together?
this sounds like typical 60Hz grounding hum to me
posted by cosmicbandito at 11:10 PM on February 8, 2006

Tellurian, it's not the signal he's trying to identify, it's the noise.

I'm living in Russia and I don't hear anything Russian about that voice.

I can't be sure, but it sounds like ungrounded flourescent ballasts to me. I'd look into the lighting situation in your area.
posted by fake at 11:18 PM on February 8, 2006

Didn't I hear this on "Lost" tonight?
posted by Dunwitty at 2:38 AM on February 9, 2006

Yeah, it's not the signal. That's CHU in Canada, the time signal station on 7335 kHz. I was tuning around the bands to give you the idea that it is pervasive and universal across frequencies, and also to give you an idea of how massive and overpowering the noise is (CHU ordinarily booms into Boston).

cosmic, I'll look into a better grounding option. The antenna itself isn't actually grounded, per se, although I am using the antenna manufacturer's recommended coax as my lead from the outside. Since the antenna has an active power supply, I should probably also try running the powering unit on the batteries instead of plugged into the grid.

fake, the fluorescent idea sounded possible to me too - that's why i mentioned the dimmers as a possible light-related issue.

These are both possibilities, in my mind. Anything else?
posted by mykescipark at 2:43 AM on February 9, 2006

Anything else?

C. Crane has a list of common AM radio noise sources and a list of steps to identify the source of radio noise at www.ccrane.com/radionoise.
posted by RichardP at 5:34 AM on February 9, 2006

It is highly likely that this noise is actually coming in through your power cord. One of the filters such as in the last link can really help. We did a huge wiring project in the house and as part of that I had a dedicated line back to the breakers put in for my stereo with hospital grade plugs and grounding and on an opposite side of the tree from motors and the like. In addition to quieting everything down it eliminated a buzzing I was getting across a wide frequency band on my tuner. A filter achieves much the same effect. If it is coming in through the air and you can identify at least the direction it is coming from you might be able to shield the antenna with foil from any signal coming from that direction.
posted by caddis at 6:22 AM on February 9, 2006

I'm totally with the 60Hz crowd. You could try a new power supply or filters on the outlet or plug. It reminds me very much of the sounds I've heard over the years on my own shortwave radio.

What radio do you use?
posted by Mo Nickels at 7:22 AM on February 9, 2006

Sounds like a ground problem to me. That or someone is running a large electrical motor nearby.
posted by Thorzdad at 8:00 AM on February 9, 2006

Sounds like a ground problem to me.

Seconded. What is your ground like?

Do you have a resistor on the loop, that can also help with interference?
posted by Pollomacho at 10:35 AM on February 9, 2006